Hopefully this will make you feel better, at least by comparison. I freelanced very often at two adjacent studios 20+ years ago. I was already booked at one, so the other hired a new assistant in town who had graduated from RIT in Rochester, NY. He assured them he knew how to load the MF film backs, and he set up a number of backs for a shoot for a large department store chain. The photographer went through three different shots before he convinced himself that there was something really wrong with the way the film had been loaded and opened a back to find that he'd been shooting on the backing paper of 120 film. He had to reshoot the group he had on set, and had to call back 10 models for the other two shots he'd finished at his expense, about $70 to $100 an hour for each model.

I know you lost shots you really wanted, but at least it's not costing you a lot of money.

The first studio I worked in tested my 4x5 loading before hiring me. I had to clean and load a dozen 4x5 holders and the photographer then checked the film orientation in every one before he would hire me. That was after I assured him that I could load film properly. When we shot his Hasselblads for the first time, I told him I'd not loaded one before, so I did some practice rolls until we were both satisfied.

The worst thing I did (for the same studio with the reversed film problem) was lose a roll of exposed 120 shooting on location in a very crowded knick-knack / novelty shop. We searched high and low and never found it.

Lee